1. Field of the Invention
The present invention relates, generally, to systems and processes capable of filtering a single tone signal, and in particular embodiments, to systems and processes capable of digitally filtering a single tone frequency modulated digital signal to generate a signal that can be subsequently filtered to remove noise mixed into the signal during transmission.
2. Description of Related Art
Modern video signal processing systems often utilize digital signal processing due to the increasing prevalence of digital video sources such as computing devices or digital video disk players. In addition, modern video signal processing systems may combine audio, video, and graphics for viewing on a video display device. In such multi-media systems, graphics information may need to be integrated into the audio and video information present within an analog video signal. Integrating graphics information into a video signal is often more easily accomplished in the digital domain. However, although a video signal may be in digital form, it often must be encoded back into an analog form compatible with typical video display devices, and then communicated to those devices. During this signal transmission, noise may be introduced into the analog video signal.
There are several different standardized formats for the analog video signal. One such format is National Television System Committee (NTSC), which is used in the United States and Japan. Another is Phase Alternation Line (PAL), which is used in Great Britain and Europe. A third is Sequentiel Couleur avec Memoire (SECAM), which is used in France, Russia and other parts of Europe.
As illustrated in FIG. 1, within an analog video signal is a single xe2x80x9clinexe2x80x9d 10 of analog video information. A line 10 is typically comprised of a front porch 12, a horizontal synchronization pulse (Hsync) 14, a subcarrier burst 16, and serial pixel data 18.
Subcarrier burst 16 is a sample of the reference subcarrier used to modulate the color information and generate chrominance signals within serial pixel data 18. Color information is comprised of two components, U and V. If U and V are zero, there is no color component to the video signal, just brightness ranging from white to gray to black. If the U or V values are positive or negative, the video signal will have color. U and V are color difference signals derivable from red (R), green (G), and blue (B) color space, from which all colors can be generated by varying the weights of R, G, and B. U and V color components and the associated luminance component, Y, can be computed from RGB color space as follows:
U=Yxe2x88x92Bxe2x80x2
V=Yxe2x88x92Rxe2x80x2
Y=0.299Rxe2x80x2+0.587Gxe2x80x2+0.114Bxe2x80x2.
The primes on R, G, and B indicate that R, G, and B are gamma-corrected, a nonlinear adjustment applied to R, G, and B because of the nonlinearity of the response of display device phosphors.
For NTSC or PAL, the U and V color components are xe2x80x9cquadrature amplitude modulated.xe2x80x9d In such a modulation system, one of these color components is multiplied by a sine representation of the subcarrier, while the other color component is multiplied by a cosine representation of the subcarrier (the same signal, but shifted by 90 degrees). These two signals are then added together to form a composite chrominance signal. For NTSC and PAL, the chrominance signal is xe2x80x9camplitude modulatedxe2x80x9d because the amplitude of the subcarrier is modified based on the U or the V information, and is xe2x80x9cquadraturexe2x80x9d because the two signals that form the chrominance signal are 90 degrees out of phase. To recover the U and V color components, the composite signal is multiplied by a sine version of a generated reference subcarrier (re-created by phase-locking a frequency source at the subcarrier burst rate to subcarrier burst 16), and is also independently multiplied by the cosine version of the generated reference subcarrier. By low pass filtering these two signals and applying trigonometric identities to the signals, the original U and V color components can be recovered. One line of serial pixel data 18 is shown in FIG. 1 as a composite sinusoidal signal having a time-varying DC component. The luminance information of the color signal is contained within the time-varying DC component of serial pixel data 18, while the chrominance information is contained within the sinusoidal signal.
Unlike NTSC and PAL, SECAM uses frequency modulation, where the frequency of the subcarrier is adjusted according to the amplitude of the color components U or V. Each line in a composite SECAM color signal will include luminance information (known as the Y component) and either U or V chrominance information, but not both. The chrominance information will consist of the frequency modulated U or V color component, referred to as Db or Dr, respectively. Thus, for each pixel in any particular line, there will be a single tone, frequency modulated signal associated with either the U or V color component. Single tone signals may be defined as signals having a single frequency at any point in time, although the frequency of such a signal may change over time, such as in a frequency modulated (FM) signal.
As with NTSC and PAL, the luminance component of a composite SECAM signal is contained within the time time-varying DC component, while the chrominance information is contained within the sinusoidal signal. Because the SECAM signal is frequency modulated, the sinusoidal signal is initially of uniform amplitude. However, there may be some variation in the amplitude if preemphasis filtering is applied after the frequency modulation. Preemphasis filtering helps eliminate noise that gets mixed into analog video signals as they are transmitted. At the receiving end, an inverse of the preemphasis filter is applied to the received signal to reject noise picked up outside the bandwidth of the analog video signal.
Conventional preemphasis filters are multi-tap filters with a frequency response in accordance with a weighted sum of the taps (different coefficients are used for each tap). Such filters typically have long pipeline delays. If a constant frequency signal is passed through the filter, the signal will be amplified in accordance with the filter""s frequency response. However, if a variable frequency signal is passed through the filter, the resultant amplitude will be a weighted average of the frequency responses of the filter to the different frequencies passing through the filter. The response of the filter is therefore relatively slow and degraded by the responses to other frequencies over time. Furthermore, conventional preemphasis filter designs introduce anomalies associated with the ringing of a step response.
Additionally, the preemphasis filter is specified in terms of a complex frequency response which extends beyond the frequency range of the signal. A conventional preemphasis filter designed to meet SECAM specifications would amplify frequencies that carry no signal more than they amplify the frequency range of the signal. Thus, amplification outside the frequency range of interest may be as much as 20 db, resulting in significant amplification of quantization noise outside the range of interest.
SECAM-formatted video signals may be operated at different pixel rates. Because the frequency response of a preemphasis filter will vary depending on the pixel rate, multiple sets of programmable coefficients are needed for conventional preemphasis filters in systems designed to support multiple pixel rates. Selecting a set of multiple coefficients to address all the frequency ranges necessary, or alternatively, implementing a filter of actual multipliers instead of hard coded optimized coefficient values, would be both space-inefficient and time consuming.
A signal processing system and process for digitally filtering a single tone digital signal is disclosed. The system includes a single tone signal generator, which may or may not perform frequency modulation. The single tone signal generator receives an input signal and generates a frequency indicator/signal which is used internally by the single tone signal generator and is also communicated to a direct realization filter. The direct realization filter uses the frequency indicator to generate a phase offset indicator/signal, which is communicated back to the single tone signal generator. The single tone signal generator uses the frequency indicator and the phase offset indicator to generate a phase-adjusted single tone signal. The direct realization filter generates a filter gain and multiplies the single tone signal with the filter gain to produce a filtered single tone signal.